1. Field of the Invention
This invention relates to a seat slide adjuster for vehicles, particularly a seat slide adjuster that is highly rigid and light in weight.
2. Description of the Prior Art
As illustrated in FIG. 1, a seat 1 for a vehicle comprises a seat back 2 and a seat cushion 3 and is supported on a vehicle body by a seat slide adjuster. It is required that the seat slide adjuster 4 be light in weight in view of reducing fuel consumption, of high strength for the sake of safety, and of high rigidity to provide a seated individual with a feeling of stability. A typical conventional seat slide adjuster of this kind is disclosed in the specification of U.S. Pat. No. 2,970,015 and is illustrated in FIGS. 2 and 3. The seat slide adjuster 4 comprises an upper rail 6 secured to a seat cushion frame 5, and a lower rail 7 secured to the vehicle body. As shown in FIG. 3, the upper rail has a generally C-shaped cross section and includes flange portions 8 on both sides, and the lower rail 7 has a generally U-shaped cross section and likewise includes flange portions 9 on both sides. The lower rail 7 is inserted into the upper rail 6 longitudinally thereof in such a manner that the flange portions 8, 9 oppose each other. Small balls 10 are arranged between these opposing flange portions, and rollers 11 are disposed between the upper and lower rails 6, 7. The rollers 11, as well as the small balls 10, are located at the front and rear and, hence, are spaced apart from each other, as shown in FIG. 2. Since sliding movement of the upper rail 6 relative to the lower rail 7 in the longitudinal direction causes the small balls 10 and rollers 11 to roll and act as bearings, the upper rail 6 has very little sliding resistance.
A downwardly directed load acting upon the upper rail 6 is received by the rollers 11, an upwardly directed stripping load is received by the small balls 10 at upper and lower points thereon, and a lateral load acting on the upper rail 6 is received by the small balls 10 at left and right points thereon. This assures that the upper rail 6 will slide smoothly with respect to the lower rail 7. However, in the state shown in FIG. 2, when a heavy individual sits down in the seat or leans back on the seat back, or when the seat back is subjected to a large load as may be sustained in a collision, the rails 6, 7 flex and may even bend and, hence, may no longer assure the necessary safety. This problem derives from the fact that the rails 6, 7 have but a small height-wise dimension and therefore exhibit a low section modulus.
A conventional attempt to solve the aforementioned difficulty is disclosed in the specification of Japanese Patent Application Laid-Open (Kokai) No. 57-90225. In this example of the prior art, a first rail having a generally I-shaped cross section is combined with a second rail having a generally C-shaped cross section, rollers are arranged at the top and bottom of the first rail and receive upwardly and downwardly directed loads, and a lateral load is received by bringing protuberances located at the sides of the rollers into contact with the inner wall surface of the second rail. However, this arrangement is disadvantageous in that the protuberances increase the sliding friction of the rails and cause looseness ascribable to wear.